In addition to the coenzyme function of retinal in vision
another vitamin A derivative, retinoic acid, is an important
hormone with effects on differentiation of cells and
tissues. It acts to control transcription of the genetic messages
in DNA by binding to specific protein receptors that
in turn bind to specific nucleotide sequences of the DNA.
The retinoid receptor proteins are a member of the steroid
hormone receptor family. Also related to this family are
receptors for hydroxylated derivatives of vitamin D.

Vitamin D can be viewed as a prohormone which arises
by the action of ultraviolet light in the two-step process
pictured in Fig. 20. Irradiation of 7-dehydrocholesterol
in the skin can provide adequate amounts of vitamin D3 (cholecalciferol or calciol). The closely related vitamin D2 (ergocalciferol) arises from irradiation of the plant sterol
ergosterol. This form of the vitamin has been widely used
in fortification of milk. However, the natural vitamin D3 is more active in preventing rickets. The term vitamin D1 was dropped when it was found to be a mixture of D2 and
D3. The principal function of vitamin D is in the control
of calcium metabolism. This control is exerted by polar,
hydroxylated compounds of which the most important is
1α, 12-dihydroxyvitamin D3 (calcitriol). This hormone is
distributed to all parts of the body. In cells of the intestinal
lining it promotes uptake of calcium ions. It promotes
reabsorption of both calcium and phosphate ions in the
kidney tubules and increases blood calcium and depositon
of calcium ions in bone.